1. Field of the Invention
The present invention relates generally to an offset voltage trimming circuit, and more particularly, to an offset voltage trimming circuit for adjusting the offset voltage of a sense amplifier circuit for amplifying and outputting a differential voltage of an input voltage.
2. Description of the Related Art
Conventionally, a sense amplifier circuit offset voltage trimming circuit like that shown in FIG. 2 is known. In FIG. 2, input signals applied to a pair of input terminals 10, 11 are supplied to the bases of npn transistors Q1, Q2 which together comprise the differential circuit (sense amplifier circuit). The emitters of transistors Q1, Q2 are jointly connected and grounded via a constant-current source 12.
The respective collectors of the transistors Q1, Q2 are connected to both ends of a resistor R3 that determines the gain and at the same time are connected to one end of each of trimming resistors R1, R2 that adjust the offset voltage. The remaining ends of trimming resistors R1, R2 are connected to a pair of output terminals 13, 14, respectively, and, at the same time, are connected to power supplies Vcc via constant-current sources 15, 16.
By connecting in series diffused resistors Ra1 through Ran which have been shorted across both ends with aluminum wiring 17 and then using a laser to remove the points on the aluminum wiring marked with an "X" as shown in FIG. 3, the trimming resistors R1, R2 shown in FIG. 2 can adjust the resistance value. By adjusting this resistance value, the offset voltage for the transistors Q1, Q2 is adjusted so that the output voltage between the pair of output terminals 13, 14 becomes zero when the voltage between the pair of input terminals 10, 11 is zero.
Conventionally, the circuit shown in FIG. 4 has been used as constant-current sources 15, 16. In FIG. 4 the base of npn transistor Q11 is grounded via resistor R11, the emitter is grounded and the collector is connected to the power supply Vcc via resistor R12. Additionally, the emitter of npn transistor Q12 is connected to the base of transistor Q11 and the base of npn transistor Q12 is connected to the collector of transistor Q11, with the collector of npn transistor Q12 acting as the current output terminal 20.
If the voltage at both ends of resistor R11 is V.sub.A (with V.sub.A being equivalent to a forward voltage drop V.sub.BE between the base and the transmitter of transistor Q11), then the output current I.sub.o at the current output terminal described above can be described as follows: EQU I.sub.o =V.sub.A /R11 (1)
It should be noted that trimming resistors R1, R2 have positive temperature characteristics. That is, when the temperature rises the resistance increases. By contrast, in the constant-current source circuit shown in FIG. 4 the temperature characteristics of the output current I.sub.o is expressed in the following formula: ##EQU1##
T represents temperature, d represents the amount of increase, dV.sub.A /dT&gt;&gt;I.sub.o dR11/dT and dV.sub.A /dT is equivalent to the forward voltage drop V.sub.BE between the base and emitter of transistor Q11, so the output current I.sub.o has a temperature characteristic of approximately -2[mV/.degree. C.].
It should be noted that, in contrast to the trimming resistors R1, R2 which have positive temperature characteristics, the constant-current sources 15, 16 have negative temperature characteristics that cancel out the positive temperature characteristics of the trimming resistors.
However, it is difficult to make the two temperature characteristic absolute values equivalent and, as a result, a problem arises in that the offset voltage for transistors Q1, Q2 changes as the temperature changes.